Hemophilia B, also called second-type Hemophilia is a kind of haemorrhage. It is an X-linked, recessive hereditary disease arising due to deficiency of fibrinogen IX (FIX). Its incidence in the male is 1/30,000. Human FIX is a single-stranded glycoprotein containing 415 amino acid residues, having a molecular weight of 56 kDa. FIX can be divided into four different functional domains; in order from the N-terminus of the protein: a Gla domain (γ-carboxyl glutamic aciddomain), a growth factordomain, an activating peptide domain and a catalytic domain (or serineprotease domain). Human FIX is synthesized mainly in the liver. Its initial translation product includes a leader sequence with 46 amino acids at the N-terminus. FIX is processed into mature FIX by cleavage of the leader peptide, glycosylation and vitamin K-dependent γ-carboxylation. FIX exists in blood in as a zymogen that is activated (“FIX”) by a protease activity FIXa or by the FVII-tissue factor complex. The content of FIX in plasma in normal people is 5 μg/ml and its half-life of activity is 24 hours (refer to Chuah MKL, DesireCollen & Vanden Driessche. Gene therapy for Hemophilia. J Gene Med 2001;3:3-20).
FIX is a necessary protein factor in the processes of the endogenous blood coagulation cascade response. A complex of FIX and regulatory protein accelerates the rate of the endogenous clotting cascade response thousands of times, which makes clotting process be finished just in a few minutes. Therefore, deficiency of FIX in the human body, can lead to endless spontaneous or minutely traumatic bleeding, serious joint distortion and lameness or death due to bleeding in the bowel or skull. The human FIX gene (hFIX) was identified in 1982 at chromosome Xq27.1. hFIX is composed of 8 exons, the coding sequence is 1.383 kb, encoding 415 amino acids. (Choo K H, Goule K G, Rees D J, et al. Nature 1982;299:178-180; Kurachi K, Davie E W. Proc Natl Acad Sci USA 1982;79:6461-6464.). The normal plasma concentration of FIX protein in normal plasma is 5 μg/ml (Kaufman R J. Human Gene Therapy 1999; 10:2091-2107). Clinical treatment of Hemophilia B is confined to protein substitution treatment by blood transfusion, or by administering a replenishing FIX preparation to obtain the correct plasma concentration. However, because FIX's half-life in the body is only 24 hours, patients need repeated transfusion or administration of blood preparations toremain alive. Hemophilia B patients not only undertake heavy economic burden, but also confront a threat of infection by HIV, HBV and mad cow virus.